There is presently a need for ovenable food trays capable of withstanding high temperatures. Most conventional home ovens are calibrated to only about .+-.50.degree. F., and may reach temperature during use of up to about 450.degree. F. It is desirable that the containers do not lose impact strength, dimensional stability or discolor, so that they may be reused.
Normally, the ovenable containers are thermoformed from a polymeric material. In thermoforming, a sheet of material is preheated to a temperature sufficient to allow the deformation thereof. The sheet is then made to conform to the contours of a mold by such means as vacuum assist, air pressure assist and matched mold assist. Thermoforming is a desirable method of producing thin wall articles.
Conventional materials used to produce thermoformed articles such as food trays include polymers such as polyethylene terephthalate. U.S. Pat. No. 3,960,807 teaches a process for thermoforming articles from a composition comprising a crystallizable polyester, a crack stopping agent and a nucleating agent. The crack stopping agent improves impact strength, and the nucleating agent provides faster crystallization. Crystallization is necessary to achieve high temperature stability.
Also of interest in U.S. Pat. No. 4,463,121 which teaches thermoforming thin-walled articles from a composition consisting of polyethylene terephthalate, a polyolefin and optionally, a heat stabilizer such as a hindered phenol. This patent teaches "higher molecular weights, as measured by increased intrinsic viscosities tend to display greater strength than lower molecular weight polyesters. When working with moderate and high molecular weight polyesters, higher crystallinity is used to increase tensile strength; however, flexural properties then diminish and the polyester becomes stiff and brittle. For any particular use of polyesters, therefore, the particular composition of materials and parameters must be carefully selected". Certain phosphate salts are known stabilizers for vinyl polymers in which they act as acid acceptors. However, it is not believed to be known in the art that they are effective antioxidants for polyesters.
Applicants have now discovered that selected phosphate salts, namely CaHPO.sub.4 and Na.sub.2 HPO.sub.4, provide improved thermo-oxidative stability to articles thermoformed from polyesters described herein. Applicants have discovered an article thermoformed from a high molecular weight polyester having high tensile strength, yet having the ability to retain its flexibility and impact resistance when subjected to high oven temperatures. In these articles, it is necessary that the polyester contain repeating units from particular dicarboxylic acids(s) and a particular glycol, and have a high inherent viscosity indicating high molecular weight. In accordance with this invention, the article contains a heat stabilizer system which will prevent brittleness when subjected to high oven temperatures. Furthermore, articles thermoformed from the polyesters in accordance with this invention retain their toughness even after being subjected to very low temperatures (about -20.degree. F.) following being subjected to high oven temperatures of about 450.degree. F. Contrary to the teachings of the art with polyethylene terephthalate, crack stopping agents or impact strength-improving additives are not necessary with the present invention.